The hypothalamus links the brain to the endocrine system by producing hormones that travel in short blood vessels directly to the pituitary. The pituitary responds by secreting hormones which act on target organs to regulate body functions important in health and disease, e.g. growth, reproduction, metabolic rate, responses to stress and pain, etc. The main barrier to understanding how the brain controls, these functions, and how external and internal perturbations upset this control, has been the inability to measure hypothalamic hormones in the living animal, without using the destructive, terminal surgery needed to sample from the deeply situated vessels that transport these hormones. To solve this problem, the applicants have developed, and are successfully applying, an original technique for collecting pituitary venous blood from conscious, unrestrained horses. This requires only puncture of a subcutaneous vein for insertion of a cannula which is carefully manipulated along venous pathways, unique to the horse, until its tip lies at the outlet of the pituitary vein. Using this technique, large (4-8 ml), frequent (@ 2 min) samples, containing accurately measurable levels of hypothalamic and pituitary hormones, can be collected from normally functioning animals for at least a week. The proposed project uses this animal model to study release patterns and secretion rates of hypothalamic and pituitary hormones regulating the adrenal and thyroid axes under basal conditions and after various stresses commonly encountered by the human and other species. The effects of experimental removal and replacement of gonadal, adrenal and thyroid hormones on hypothalamic hormones secretion will be measured, for the first time in normal subjects, to investigate the central effects of widely used hormone and antihormone therapy. These experiments will test hypotheses suggested by indirect and in vitro studies. This project should provide basic information, unobtainable by any other method, on the brain's hormonal control of the body, which could find widespread application in human and veterinary medicine in areas ranging from fertility control to stress management.